Three major joining technologies exist for aerospace composite structures: mechanical fastening, adhesive bonding, and welding. Both mechanical fastening and adhesive bonding are time consuming, costly assembly steps. Mechanical fastening requires expensive hole locating, drilling, and fastener installation. Adhesive bonding requires complicated surface pretreatments and precise fit-up.
In contrast, composite welding holds promise to be an affordable joining process. It features the ability to form high strength, reliable and repeatable bonds of thermoplastic composite components at high production rates with minimal labor and little surface pretreatment. Development of a successful induction welding process would offer significant advantages over traditional mechanical fastening methods, including reduced parts count, short process cycle time, and greater ease of making rapid field repairs of composites or other structures.
Contrasted with adhesive bonding, thermoplastic welding offers another set of significant advantages, including enhanced joint performance (especially hot/wet and fatigue) and little or no loss of bond strength after prolonged exposure to environmental influences, minimal surface preparation, and indefinite shelf life at room temperature.
Thermoplastic welding is a process for forming a fusion bond between two faying thermoplastic faces of two parts to be welded together. A fusion bond is created when the thermoplastic on the surface of the two parts is heated to the melting or softening point and the two surfaces are brought into contact and held in contact while the material cools below the softening temperature.
Simple as the process sounds, and easy as it is to perform in the laboratory on small pieces, it becomes devilishly difficult to perform reliably and repeatably in a real factory on real full-scale parts to build a large structure such as an airplane wing box. The difficulty is in getting the proper amount of heat to the bondline without over heating the entire structure which could cause it to sag, and also in achieving intimate contact of the faying surfaces of the two parts at the bondline during heating and cooling despite the normal imperfections in the flatness of composite parts, thermal expansion of the thermoplastic during heating to the softening or melting temperature, flow of the thermoplastic out of the bondline under pressure, and then contraction of the thermoplastic in the bondline during cooling.
One technique for getting heat to the bondline in a thermoplastic assembly to be welded together is to include a conductive layer or article, known as a "susceptor", between the two surfaces to be welded, and to heat the susceptor by resistive heating so that the susceptor functions as a bondline heat source to melt or soften the thermoplastic at the bondline for fusion of the faying surfaces of the composite components to be joined. The electric current for heating the susceptor can be in the form of eddy currents generated inductively, as taught for example by U.S. Pat. Nos. 3,395,261 and 4,978,825, or it can be conducted directly to the susceptor through tabs or the like as shown in U.S. Pat. No. 5,313,034. One susceptor that is particularly effective for use in an inductive heating welding process is disclosed in U.S. patent application No. 08/068,520, now abandoned.
Significant effort has been expended in developing inductor and susceptor systems to optimize the heating of the bondline in the thermoplastic assemblies to be welded. Induction coil structures and tailored susceptors have now been developed that provide adequate control and uniformity of heating of the bondline, but a big hurdle remaining to perfecting the process to the point of practical utility for producing large scale aerospace-quality structures in a production environment is the aspect of the process dealing with the control of the surface contact of the faying surfaces of the two parts to be welded together, and the timing, intensity, and schedule of heat application so the material at the faying surfaces is brought to and maintained within the proper temperature range for the requisite amount of time for an adequate bond to form, and is maintained in intimate contact while the melted or softened material hardens in its bonded condition.
Large scale parts such as wing spars and ribs, and the wing skins that are bonded to the spars and ribs, are typically on the order of 20-30 feet long at present, and potentially can be hundreds of feet in length when the process is perfected for full scale commercial production. Parts of this magnitude are very difficult to produce with perfect flatness. Instead, the typical part will have various combinations of surface deviations from perfect flatness, including large scale waviness in the direction of the major length dimension, twist about the longitudinal axis, dishing or sagging of "I" beam flanges, and small scale surface defects such as asperities and depressions. These irregularities interfere with full surface area contact between the faying surfaces of the two parts and actually result in surface contact only at a few "high points" across the intended bondline. Additional surface contact can be achieved by applying pressure to the parts to force the faying surfaces into contact, but full intimate contact is difficult or impossible to achieve in this way. Applying heat to the interface by electrically heating the susceptor in connection with pressure on the parts tends to flatten the irregularities further, but the time needed to achieve full intimate contact with the use of heat and pressure is excessive, can result in deformation of the top part, and tends to raise the overall temperature of the "I" beam flanges to the softening point, so they begin to yield or sag under the application of the pressure needed to achieve a good bond.